The most significant innovation in medieval agriculture came in 3-crop rotation rather than the 2-crop of the classical Mediterranean.
In 2-crop rotation, planted fields were left fallow in alternate years, to retain soil fertility and accumulate moisture. This worked well for the light soils and long, dry summers of the Mediterranean basin. The relatively light Mediterranean plow sufficed to furrow a field.
A typical rotation might be a spring crop – barley or oats, sometimes beans or peas – harvested in summer; an autumn sowing of bread grains – wheat or rye – reaped the following summer. Then the land lay fallow for a year.
Settled agriculture was seldom practiced before Rome extended its reach into northwest Europe. The Gauls and Germanic tribes relied largely upon their cattle herds. When field crops were planted, slash-and-burn cleared the ground. A location went abandoned when soil fertility declined.
The Romans brought their agricultural practices, but their plows were impotent in penetrating the deep loam soils in the northwest, so the Romans limited themselves to sandy soils and chalky hills with adequate drainage, avoiding the heavier dirt in the more fertile plains and valleys.
The heavy-wheeled plow was able to break the thick soil. The date and place of its invention has been plowed under by the mists of time, but it was well after Roman times. The heavy plow required several oxen or other draft animals, and so contributed to cooperative cultivation.
The moist climate of northwestern Europe and richer soil meant that leaving fields fallow was unnecessary, particularly if alternate crops were planted in rotation.
The innovation of 3-crop rotation began in northern France in the late 8th century. By the beginning of the 11th century 3-crop rotation was the norm throughout northwestern Europe.
3-crop rotation yielded 33% more food and was more efficient as well. It also reduced the risk of famine from crop failure. Other innovations, including better agricultural implements, contributed.
With more land available for exploitation came greater variety. This improved nutrition.
The upshot was a burgeoning population. Western Europe had 12–15 million people in 1000. Within 300 years it was 3 to 4 times that: 45–50 million.